Southern Resident Killer Whale Prey Selectivity in Relation to Chinook Salmon Stock and Size Composition within Canadian Critical Habitat

Abstract

Chinook salmon are a key prey of Southern Resident Killer Whales (SRKW) and variability in individual traits (e.g., body size, lipid content) affects their nutritional value to SRKW. Spatiotemporal distribution, local abundance, and fish behavior may influence the accessibility and availability of Chinook salmon to SRKW. Using recreational fisheries data (2014-2023), a geostatistical model was developed to estimate the stock composition and the size composition of Chinook salmon within SRKW critical habitat in Canadian waters, during May-September, when SRKW diets are dominated by Chinook salmon. To investigate SRKW prey selectivity, a co-occurrence approach was taken to compare the size and stock composition of SRKW prey remains in relation to model estimates from recreational fishery samples, in the western portion of SRKW critical habitat from June to September (2017-2023). All Chinook salmon stocks observed in the recreational fisheries data occurred in SRKW prey remains. Fraser River Spring 52, Summer 52, and Summer 41 stocks were more common, while Puget Sound, West Coast Vancouver Island (WCVI), Columbia River Summer/Fall, and “Other” stocks were less common, in prey remains than predicted by the recreational fishery model. Chinook salmon estimated to be smaller than 75 cm fork length were rarely observed in SRKW prey remains. Chinook salmon greater than 75 cm fork length were more common, while fish between 55 and 75 cm fork length were less common than predicted by the recreational fishery model. While total terminal abundance summed across all stocks considered here has remained relatively stable since 1982, Fraser River Spring 52 and Summer 52 Chinook salmon populations have declined in abundance, while the Fraser River Summer 41 and WCVI stocks have increased in abundance. Puget Sound and Columbia River Summer/Fall abundance has varied cyclically. SRKW have poor body condition in the spring. In the current study, SRKW showed selectivity towards Chinook salmon stocks with high lipid content and Chinook salmon individuals with large body size. Therefore, improvements to the SRKW prey field in Canadian critical habitat may occur via increasing the abundance of Chinook salmon with early migration timing, high lipid content, and large body size. Independent of abundance, SRKW may also benefit from increases in prey quality as represented by the body size and/or lipid content of Chinook salmon stocks. Key sources of uncertainty in the SRKW prey remains data include potential sampling bias, as well as low precision due to a relatively small sample size. A key source of uncertainty associated with characterizing Chinook salmon available to SRKWs is the extent to which fisheries-dependent samples accurately represent the underlying prey base. The importance of Chinook salmon stocks outside of Canadian critical habitat and during other periods of the year was not considered here because prey remains samples were not available. Non Chinook salmon and other species present in SRKW diets were not evaluated and warrant additional research.